1. Field of the Invention
This invention relates to a process for the preparation of the insecticide imidacloprid.
2. Prior Art
Imidacloprid, namely, 1-(6-chloro-3-pyridylmethyl)-N-nitroimidazolidin-2-ylideneamine, is a systemic, chloro-nicotinyl insecticide with soil, seed and foliar uses for the control of sucking insects including rice hoppers, aphids, thrips, whiteflies, termites, turf insects, soil insects and beetles. It is most commonly used on rice, cereals, maize, potatoes, vegetables, sugar beets, fruits, cotton, hops or turfs, and is especially systemic when used as a seed or soil treatment.
U.S. Pat. No. 6,307,053 describes a process for the preparation of imidacloprid comprising reacting 2-nitroiminoimidazolidine with 2-chloro-5-chloromethyl pyridine in stoichiometric amounts in the presence of an alkali metal carbonate in an organic solvent under reflux condition. The alkali metal carbonate may be lithium, potassium or sodium carbonate. The organic solvent may be alcohols, ketones, acetonitrile or dimethyl formamide. Due to the alkali metal carbonates being not very soluble in the organic solvents, the solutions of the alkali carbonates are not easily flowable and pumpable, and the reaction mixture is not very homogenous so as to be easily stirred. Therefore, the above process is difficult to carry out. Besides, the filtrate containing imidacloprid obtained by the above process is concentrated by vacuum distillation. Due to the presence of unreacted alkali metal carbonates and their related salts in the filtrate resulting from use of alkali carbonates, there is bumping (flashing) turbulence during vacuum distillation of the filtrate, thereby rendering the process more difficult to carry out. Also, alkali metal carbonates have low basisity, thereby increasing the reaction time. The above process is described to be an improvement over the process of employing sodium hydride (NaH) in the place of alkali metal carbonate. Sodium hydride is a very hazardous reagent. Therefore, handling of the sodium hydride-based reaction is very difficult and cumbersome.
Indian Patent No 181755 describes a process for the preparation of imidacloprid comprising, among other steps, condensation of 3-chloromethyl-6-chloropyridine with 2-nitroimino-1,3-dihydro imidazole in the presence of inorganic bases and ketonic solvents. The inorganic bases used may be carbonates or bicarbonates of sodium or potassium. The solvents may be acetone, methyl butyl ketone, methyl-t-butylketone or acetonitrile. The problems associated with the use of alkali carbonates in the above-referenced U.S. Pat. No. 6,307,053 are encountered in this process also.